This invention relates generally to optical communication systems and, more particularly, to techniques for separating transmit and receive signals at an optical transceiver. Because transmit and receive signals are propagated over paths that are practically collinear, an approach is needed to ensure that signals transmitted from a transceiver do not corrupt or interfere with signals received at the same transceiver.
Known methods for separating and isolating transmit and receive signals include using different wavelengths or different polarization modes for the transmit and receive signals, referred to as wavelength diversity and polarization diversity, respectively. Another known technique uses time diversity to separate the signals, which is to say that the transmit and receive signals occupy different time slots and communication takes place in a simplex mode.
A critical element of the wavelength diversity approach is a dichroic beam splitter, the function of which is to reflect a receive beam but to pass a transmit beam or vice versa. The transmit beam passes through the dichroic beam splitter and proceeds along a transmission path. A receive beam is received over practically the same path and is reflected through a selected angle by the beam splitter. If the beam splitter is oriented at, say, 45° to the transmission path, the receive beam will be reflected through 90° and may then be conveniently processed in an optical receiver without interference with the transmit beam.
A similar separation of the two transmission paths can be effected by a polarization beam splitter, where the transmit beam is vertically polarized, for example, and the receive beam is horizontally polarized, but may be of the same wavelength as the transmit beam. The vertically polarized transmit beam passes through the polarization beam splitter and proceeds along the transmission path. The receive beam, being horizontally polarized, is reflected by the polarization beam splitter and provides the desired separation of the transmit and receive paths. Left and Right handed circular polarizations can also be used with the addition of a quarter wave plate to convert circular to linear polarization.
In a simplex communication link, an optical switch might be used to toggle back and forth between transmit and receive modes. In the transmit mode, the transmit beam passes through the optical switch and proceeds along the transmission path. In the receive mode, the receive beam enters the optical switch from the transmission path and is routed to the receive port while the transmit signal is routed to a dumped port. Once again, the transmit and receive signals may have the same wavelength.
All three of these prior art approaches have significant shortcomings. In order to obtain a high degree of isolation in the wavelength diversity approach, the dichroic beam splitter has to be designed to have many coating layers to effect the desired wavelength separation. Such a complex design is likely to have more insertion loss than a dichroic beam splitter of simpler design. Alternatively, the transmit and receive wavelengths may be selected to be widely spaced, but doing so may significantly limit the number of wavelengths that can be used within a limited optical amplifier bandwidth. The dichroic beam splitter may be designed with this trade-off between splitter complexity and wavelength separation in mind, but the wavelength diversity approach always requires some combination of design complexity and wide wavelength separation to produce a desirably high degree of isolation between the transmit and receive beams.
In the polarization isolation approach, the polarization beam splitter provides isolation performance of typically around 20 to 30 deciBels (dB), or 40 dB at best. The isolation required for ultra-long distance laser communication is, however, greater than 110 dB. Therefore, using polarization diversity for isolation also requires the use of other means to provide additional isolation. An in-fiber filter can typically provide an additional 60 dB of isolation, but this might not be sufficient for some applications. Depending where this filter is installed in relation to an optical low-noise amplifier, additional losses may be incurred as a result of the filter's use. Another drawback of polarization isolation is that photonics components using required polarization in space communications may be difficult to obtain and qualify for use.
Finally, the simplex approach is the most straightforward but, of course, has inherent limitations in comparison with a full duplex communication system. In addition, an optical switch approach provides around 50 dB of isolation. Other means of isolation will be required.
It will be appreciated, therefore, that there is still a significant need for an alternative approach to separation of transmit and receive beams in an optical communication system. The present invention satisfies this need.